This invention relates to color mixing in gas discharge lamps and more specifically to a system and method for determining the frequency of a second longitudinal mode signal provided to a gas discharge lamp.
High intensity discharge lamps (HID) are becoming increasingly a popular because of their many advantages, such as high efficacy and brightness, compared to other types of low pressure mercury vapor flourescent lamps. These HID lamps can be driven by a high frequency electronic ballast that is configured to generate driving current signals at above 20 kHz range.
A major obstacle to the use of high frequency electronic ballasts for HID lamps, however, is the acoustic resonances/arc instabilities which can occur at high frequency operation. Acoustic resonances, at many instances, can cause flicker of the arc which is very annoying to humans. Furthermore, acoustic resonance can cause the discharge arc to extinguish, or even worse, stay permanently deflected against and damage the wall of the discharge lamp.
Recently, a new class of high intensity discharge lamps has been developed that employ ceramic (polycrystalline alumina) envelopes. The discharge envelope in this class of lamps is cylindrical in shape, and the aspect ratio, i.e., the inner length divided by the inner diameter is close to one, or in some instances more than one. Typically, lamps with aspect ratios much greater than one have the desirable property of higher efficacy, but they have the disadvantage of having different color properties in vertical and horizontal operation. In particular, in vertical operation color segregation occurs.
The color segregation can be observed by projecting an image of the arc onto a screen, which shows that the bottom part of the arc appears pink, while the top part appears blue or green. This is caused by the absence of complete mixing of the atomic metal additives in the discharge. In the upper part of the discharge there is excessive thallium and mercury emission and insufficient sodium emission. This phenomena leads to higher color temperature and decreased efficacy compared to horizontal operation.
U.S. application Ser. No. 09/335,020 entitled Reduction of Vertical Segregation In a Discharge Lamp, filed Jun. 17, 1999, and incorporated herein by reference, now U.S. Pat. No. 6,184,633 teaches a method to eliminate or substantially reduce arc instabilities and color segregation, by providing a current signal frequency sweep within a sweep time, in combination with an amplitude modulated signal having a frequency referred to as second longitudinal mode frequency. The typical parameters for such operation are a current frequency sweep from 45 to 55 kHz within a sweep time of 10 milliseconds, a constant amplitude modulation frequency of 24.5 kHz and a modulation index of 0.24.
The modulation index is defined as (Vmaxxe2x88x92Vmin)/(Vmax+Vmin), where Vmax is the maximum peak to peak voltage of the amplitude modulated envelope and Vmin is the minimum peak to peak voltage of the amplitude modulated envelope. The frequency range of 45 to 55 kHz is between the first azimuthal acoustic resonance mode frequency and the first radial acoustic resonance mode frequency. The second longitudinal mode resonance frequency is then derived mathematically, where the power frequency of the nth longitudinal mode resonance is equal to n*C1/2L where n is the mode number, C1 is the average speed of sound in the axial plane of the lamp and L is the inner length of the lamp. However, it is not possible to calculate accurately the second longitudinal mode resonance frequency of a given lamp, because of small variations in C1 caused by temperature variations in the lamp. In addition, small differences in L can occur because of manufacturing tolerances.
Therefore, despite the remarkable teachings of the above-identified application, there is a need for an improved system and a method to determine the second longitudinal mode frequency of the signal that is provided to the discharge lamp in a convenient and accurate manner.
In accordance with one embodiment of the invention, a high intensity discharge lamp is operated through a current frequency sweep within a sweep time, in combination with an amplitude modulated signal having a frequency corresponding to the second longitudinal acoustic resonance mode of the discharge lamp. The second longitudinal mode frequency, fY, is derived by first setting a lower limit second longitudinal mode frequency fL, and an upper limit second longitudinal mode frequency fH. The gas discharge lamp is then provided with a current signal that has a frequency sweep ranging between the first azimuth acoustic resonance mode frequency and the first radial acoustic resonance mode frequency, respectively corresponding to the first azimuthal acoustic resonance mode of the lamp and the first radial acoustic resonance mode of the lamp.
The frequency swept current signal is then amplitude modulated with a signal having frequency fH and a specified modulation index a. The lamp voltage is then measured. The amplitude modulated frequency fH is then decreased by a specified amount xcex94f and then mixed again with the frequency swept current signal. The lamp voltage is repeatedly measured until the frequency of the amplitude modulated signal reaches fL. A frequency vs. voltage curve is generated from fH to fL kHz The maximum in the lamp voltage corresponds to the frequency fmax, which is then utilized for color mixing.
In accordance with another embodiment of the invention, the amplitude modulated signal is turned xe2x80x9coffxe2x80x9d following the application of the amplitude modulated signal and the lamp voltage measurement.
In accordance with yet another embodiment of the invention a background subtraction mechanism is employed to compensate for short term fluctuations of the lamp voltage that are independent of amplitude modulation and color mixing effects. To this end, the lamp voltage measurements are performed such that the lamp voltage signal values with amplitude modulation xe2x80x9coffxe2x80x9d before and after mixing the amplitude modulated signal with the swept frequency signal, are averaged and subtracted from the lamp voltage with amplitude modulation mixing xe2x80x9con.xe2x80x9d
Once the color mixing frequency fmax is determined, the frequency swept signal is mixed with an amplitude modulated signal having a frequency fH again. The amplitude modulation frequency is then decreased to frequency, fmax and the modulation index a is increased to amix, which is a modulation index to be employed in color mixing mode.
In accordance with yet another embodiment of the invention an HID lamp is driven by two separate signals in a time sequential arrangement. Thus, the first signal comprises a fixed frequency signal at half the frequency of the second longitudinal mode resonance, followed by the second signal comprising a frequency sweep between the first azimuthal acoustic resonance mode and the first radial acoustic resonance mode. The second longitudinal mode frequency, fY, is derived by first setting a lower limit second longitudinal mode frequency fL, and an upper limit second longitudinal mode frequency fH.
The lamp is driven in a time sequential arrangement by first providing a fixed frequency signal having a frequency fH//2 for a fixed period of time, x, followed by a frequency sweep for a period of time (t-x). The lamp voltage is then measured. The fixed frequency signal is then turned xe2x80x9coff,xe2x80x9d by increasing the sweeping frequency signal for the full period of time, the fixed frequency fH is then decreased by a specified amount xcex94f and then applied to the lamp again with the frequency swept current signal. The lamp voltage is repeatedly measured until the frequency of the fixed frequency signal reaches fL/2. A frequency vs. voltage curve is generated from fH to fL kHz. The maximum in the lamp voltage corresponds to the frequency ftsmax, which is then utilized for color mixing.